In the processing of semiconductor substrates (e.g., wafers), plasma is often employed. In plasma processing, the wafers are processed using a plasma processing system, which typically includes a plurality of processing modules. The substrate (e.g., wafer) is disposed on a chuck inside a processing module during plasma processing.
In order to move a wafer in and out of the processing module, the wafer is typically placed on an end effector and transferred onto the chuck. The end effector is a structural component configured for supporting the wafer during wafer transfer. The end effector is typically disposed on a robot arm. FIG. 1 shows a representative prior art end effector 102 for supporting a wafer 104 during wafer transfer. For illustration purposes, a portion of a robot arm 106 is also shown.
Generally speaking, during a wafer transfer sequence, the robot arm first moves the end effector to pick up the wafer from a wafer storage cassette or station. Once the wafer is positioned on the end effector, the robot arm would then move the wafer into the plasma processing module through a door in the processing module. The robot arm then positions the end effector and the wafer over the chuck and then places the wafer on the chuck for plasma processing.
In order to ensure that the wafer is processed properly (thereby ensuring controllable and repeatable process results); the wafer needs to be centered on the chuck during plasma processing. If the end effector is correctly centered relative to the chuck and the wafer is correctly centered relative to the end effector, then the wafer would be correctly centered relative the chuck when the robot arm places the wafer on the chuck. However, for many reasons, some of which are discussed below, this ideal scenario is rarely the case.
Due to machining and/or manufacturing tolerances between the various components of the processing chamber, it is possible that the center defined by the end effector (herein referred to as the “end effector center” or the “end effector-defined center”) is slightly offset relative to the center of the chuck in a given processing module. As a result, it is possible that the center defined by the end effector may not be correctly aligned with the center of the chuck at the robot arm position that the robot controller deems to be the correct position for wafer placement. If this end effector/chuck mis-alignment is not compensated for during production, the wafer may be inaccurately placed relative to the chuck center during wafer processing. In a co-pending patent application entitled “SYSTEMS AND METHODS FOR CALIBRATING END EFFECTOR ALIGNMENT IN A PLASMA PROCESSING SYSTEM,” Ser. No. 12/810,776, filed on even date herewith by the inventors herein and incorporated herein by reference, techniques have been proposed to address this end effector/chuck mis-alignment.
However, even if the end effector center is correctly aligned with the chuck center (or can be made to achieve the effect of a correct alignment), there exists another potential source of error that may result in wafer/chuck mis-alignment during production. That is, different production wafers may be positioned on the end effector differently. If the end effector center is not correctly or consistently aligned with the center of the wafers, wafer/chuck mis-alignment may still occur during production. In this case, even though the end effector center is correctly aligned with the chuck center, the wafer/end effector mis-alignment will cause the wafer to be offset relative to the chuck when the end effector deposits the wafer on the chuck for processing.
Unlike the end effector/chuck misalignment problem, which tends to be a consistent error for all wafers in a given processing module since that alignment error arises from chamber component tolerances and robot calibration issues, the wafer/end-effector mis-alignment may vary with each production wafer. In other words, each production wafer may be positioned on the end effector differently, resulting in differences in the mis-alignment. Accordingly, the solution to address such end effector/wafer misalignment requires a dynamic approach, i.e., one that can adjust for the error of each individual production wafer relative to the end effector during production.
In the prior art, the end effector/wafer mis-alignment is addressed using a dynamic alignment beam approach. A dynamic alignment (DA) beam detection system typically employs two beams (i.e. laser beams) located at the entrance of the plasma processing module door. As the wafer moves through the DA beams (with the beams being orthogonal to the wafer translation plane), the DA beams are broken as the wafer enters the beam, and then resumed at the point where the wafer is no longer present. This pattern of beam signal break-then-make generates a production DA beam pattern.
In the dynamic alignment beam approach, it is necessary to obtain a reference DA beam pattern, i.e., the DA beam pattern that is generated when a wafer that is correctly centered on the end effector moves through the DA beams. By comparing the production DA beam pattern (i.e., the beam pattern obtained for a production wafer) with the reference DA beam pattern, an error vector may be obtained. The robot controller can them move the robot arm by the requisite amount to correct for the end effector/wafer mis-alignment during production. Further information regarding dynamic alignment beams may be found in, for example, issued U.S. Pat. Nos. 6,502,054 and 6,629,053, incorporated herein by reference.
The process of obtaining a reference DA beam pattern is referred to herein as DA beam calibration. In order to calibrate the DA beams, it is necessary then to acquire or obtain a DA beam calibration assembly that includes a wafer correctly centered on the end effector and to move that DA beam calibration assembly through the DA beams so a reference DA beam pattern can be acquired.
In the prior art, the DA beam calibration assembly is obtained using a fabricated disk, which simulates a water. The disk has a downward protruding flange that fits on a notch of the end effector (such as notch (not shown) of end effector 102 in FIG. 1). Once the disk is fitted to the notch of the end effector, this combination simulates a correctly centered wafer with respect to end effector. The combination of simulated wafer/end effector is then moved by the robot arm into the processing module in a straight line trajectory path toward the chuck through the DA beams in order to obtain a reference DA beam pattern.
However, there are disadvantages with the prior art technique of using a wafer-simulating disk to create a calibration assembly for the purpose of obtaining a reference DA beam pattern. First of all, attaching a physical mechanical fixture (such as the wafer-simulating disk) on the end effector may potentially damage the end effector.
Additionally, if this calibration is done in the field after some plasma cycles have been executed in the processing module, the placement of a physical or mechanical fixture on the end effector may cause deposited particles on or near the end effector to flake off into the processing module. During the subsequent processing cycles, such particles constitute particle contamination, which is undesirable.
Additionally, because the calibration is performed at atmospheric pressure, the prior art calibration technique may not effectively duplicate the conditions that exist during production. This is because during production, components of the processing module may be placed under vacuum, causing one or more components to shift due to the differential in pressures between the vacuum environment and the ambient atmosphere. Since the calibration conditions do not faithfully duplicate the production conditions, accurate calibration may not be possible. If the calibration process is inaccurate, inaccurate wafer placement during production may occur, leading to decreased yield and an increase in the product rejection and/or failure rate.